1. Field of the Invention
The present invention relates to a land pre-pit address demodulating device, which detects a land pre-pit address from light reflected by a DVD-R/RW disc, and more particularly to a land pre-pit address demodulating device for use in an optical disc recording apparatus which records data to the DVD-R/RW disc.
2. Description of the Background Art
DVD-R/RW is a format for recording/reproducing information to/from an optical disc. A first feature of the DVD-R/RW format is that in order to increase compatibility with DVD-ROM format, address information, which is required for specifying an address where information is recorded or reproduced, is formed in a space portion (also referred to as a “land”) between guide grooves (also simply referred to as “grooves”) of a disc on which information recording or reproduction is performed. The address as described above is referred to as a “land pre-pit (LPP) address”. The term “LPP” refers to a pit which is characteristic of the DVD-R/RW format and represents an address formed in a wobble groove for indicating location information for disc recording, and the LPP address refers to location information represented by an LPP.
The optical disc has an area called “track” where information is recorded as a mark. When following the track, a tracking detector provided in an optical disc apparatus detects incident light and generates a plurality of light intensity signals. Address information is detected based on a differential signal obtained via subtraction between the plurality of light intensity signals, while information to be recorded/reproduced is detected based on an addition signal obtained via addition of the plurality of light intensity signals.
A second feature of the DVD-R/RW format is that wobbles are formed in the optical disc such that the guide groove wiggles at a prescribed frequency in a radial direction. A wobble signal, which is obtained based on the wobbles, is used as a reference signal for generating a clock signal for information recording and reproduction. Similar to the address information, the wobbles are detected based on a differential signal obtained via subtraction between a plurality of light intensity signals.
In an optical disc recording/reproduction apparatus, in a simplistic sense, recording of information to a DVD-R/RW disc is performed by irradiating the track with a recording laser beam which alternately changes its intensity between high and low. Specifically, a high intensity laser beam is irradiated to heat a recording coating so as to become amorphous, and thereafter the heated recording coating is rapidly cooled, thereby forming a low reflective mark. Then, a low intensity laser beam is irradiated to crystallize the recording coating, thereby forming a high reflective space. In this manner, a mark is formed as a pit on the track between two spaces.
A tracking detector provided on an optical head is divided into four portions by two divide lines respectively parallel and perpendicular to a direction along the track. In other words, there are four tracking detectors A, B, C, and D along a circumferential direction of the optical disc. Note that the tracking detectors A and D are divided from the tracking detectors B and C by the divide line parallel to the track. Each of the four tracking detectors A, B, C, and D detects the intensity of a laser beam irradiated by the optical head and reflected by the optical disc, and outputs the detected intensity of the laser beam.
The tracking detectors A, B, C, and D output light intensity signals Ta, Tb, Tc, and Td, respectively. The light intensity signals Ta and Td are added to become a TE+ signal, and the light intensity signals Tb and Tc are added to become a TE-signal. That is, the tracking detectors A and D and the tracking detectors B and C can be considered as being two tracking detectors AD and BC divided by the divide line parallel to the track. A difference between the TE+ and TE− signals respectively outputted from the two tracking detectors AD and BC forms a tracking error signal TE. This is represented by TE=(TE+)−(TE−).
Referring to FIGS. 12, 13, 14, 15, and 16, described next is a land pre-pit address demodulating device included in a conventional optical disc recording/reproduction apparatus proposed in Japanese Patent Laid-Open Publication No. 2002-216363. As shown in FIG. 12, a land pre-pit address demodulating device DMc includes a first input terminal Ti1, a second input terminal Ti2, a subtractor 1, a comparator 5c, and an output terminal To. The first and second input terminals Ti1 and Ti2 are respectively connected to the above-mentioned tracking detectors AD and BC, such that the TE+ and TE− signals are inputted to the first and second input terminals Ti1 and Ti2, respectively.
FIGS. 13 and 14 showwaveforms of the TE+ and TE− signals, respectively. As shown in FIG. 13, the TE+ signal includes: a radio frequency (RF) component Erf an envelope of which varies sinusoidally; and an LPP component Elpp at each peak of the RF component Erf. Note that the envelope of the RF component Erf represents a wobble component Ewbl. As shown in FIG. 14, basically, the TE− signal has a waveform similar but opposite in sign to the waveform of the TE+ signal. Whether a mark portion or a space portion is on the high potential side depends on properties of an optical pickup to be used. In examples shown in FIGS. 1 and 2, the mark portion is on the high potential side.
In general, the term “mark” refers to a pit portion formed on a disc, and the term “space” refers to a region between pits. In the present specification, however, in order to avoid redundancy in description, the term “mark” may also refer to a voltage level corresponding to the mark portion among voltage levels of an electric signal into which a detector converts light reflected by the disc, and the term “space” may also refer to a voltage level corresponding to the space portion among the voltage levels of the electric signal.
The subtractor 1 is connected to the first and second input terminals Ti1 and Ti2 and subtracts an inputted TE− signal from an inputted TE+ signal, thereby generating a tracking error signal TE to be outputted therefrom.
FIG. 15 shows a waveform of the tracking error signal TE. In principle, by subtracting the TE− signal from the TE+ signal, it is possible to remove an RF component (i.e., a signal portion corresponding to the mark and the space) and thereby to extract the wobble component Ewbl and the LPP component Elpp. In FIG. 15, the wobble component Ewbl corresponds to a low frequency, and the LPP component Elpp corresponds to a pulse at a peak of the wobble component Ewbl.
The comparator 5c has two input ports respectively connected to the subtractor 1 and a reference potential Vref. Specifically, the comparator 5c binarizes the tracking error signal TE, which is inputted from the subtractor 1, using levels of the reference potential Vref as an LPP binarization level L1 and a WBL binarization level Lw, thereby generating an LPP binarized signal Bp to be outputted from the output terminal To.
FIG. 16 shows waveforms of various signals obtained by adjusting a level at which to binarize the tracking error signal TE. In FIG. 16, dotted line Lw represents the WBL binarization level Lw which is used as a threshold value for binarizing a wobble signal component of the tracking error signal TE, and rectangular wave Bw represents a WBL binarized signal Bw to be obtained. Similarly, dotted line L1 represents the LPP binarization level L1 which is used as a threshold value for binarizing an LPP signal component of the tracking error signal TE, and rectangular wave Blpp represents an LPP binarized signal Blpp. Note that the LPP binarized signal Blpp is also referred to as an LPP detection signal, and the WBL binarized signal Bw is also referred to as a wobble detection signal.
The tracking error signal TE shown in FIGS. 15 and 16 is obtained in an on-track state where the center of a photodetector corresponds to the center of a disc groove. There is substantially no difference between the TE+ and TE− signals with respect to voltage levels of the mark portion and the space portion. In each of the TE+ and TE− signals, there is a considerable difference between maximum and minimum voltage levels of each of the mark portion and the space portion. Accordingly, in the tracking error signal TE, portions of the RF component Erf, which correspond to the mark portion and the space portion, respectively, are clearly removed, and therefore LPP components Elpp are highly noticeable. Here, the height of an LPP component Elpp projecting from the wobble component Ewbl is referred to as an “on-track LPP height Ha”.
FIG. 17 shows a waveform of a tracking error signal TE obtained in an off-track state. Specifically, in a real optical disc recording/reproduction apparatus, the TE+ and TE− signals become nonuniform in amplitude depending on, for example, sensitivity of a light receiving element of the optical pickup, an impedance of a flexible wire conductor connecting the optical pickup to an IC chip, and a gain difference caused due to element-to-element variation between input amplifiers on the IC chip. As a result, portions of the RF component are left unremoved when the TE− signal is subtracted from the TE+ signal. In FIG. 17, the unremoved portions of the RF component correspond to portions between two sinusoidal waves. Hereinafter, the residue of the RF component is referred to as an “RF residual component Rrp”.
The height of an LPP component Elpp projecting from a peak of the wobble component Ewbl is lower than the on-track LPP height Ha by a height of the RF residual component Rrp. The height of the LPP component Elpp in this case is referred to as an “off-track LPP height Hb”. That is, a relationship Ha>Hb is established. Accordingly, even if the binarization level is determined as in the case of the on-track mode described in conjunction with FIG. 16, the LPP binarized signal Bp or the WBL binarized signal Bw cannot be correctly detected. Therefore, in the above-described conventional optical disc recording/reproduction apparatus, a gain adjustment is performed such that the RF component Erf has a uniform amplitude, in an attempt to increase a detection rate of the LPP.
However, in high-speed recording, an irradiation power per unit area of a recording laser beam is small, i.e., heat dissipation from a disc surface is increased during the high-speed recording, and therefore in order to form a mark similar to that generated in a low-speed recording, it is necessary to increase the irradiation power of the laser beam. In general, it is known that when a recording speed is doubled, the irradiation power of the laser beam is required to be increased to 1.41 times the normal irradiation power. On the other hand, it is known that the amount of a change in film quality of a disc is small in comparison to a change of the laser power, and therefore it is not necessary to increase the laser power in order to form a space portion during high-speed recording.
As described above, in the case of multiple-speed recording, i.e., high-speed recording, which is performed at a speed higher than a standard recording speed, the laser power is increased in order to form a mark portion. As a result, in each of the TE+ and TE− signals, the amplitude of light components reflected by the mark portion is increased, while the amplitude of light components reflected by a space portion is relatively decreased. In order to cancel the changes of the amplitudes of the light components reflected by the mark portion and the space portion, it is necessary to reduce a gain to the same level as an amplitude level in a low-speed recording, with consideration of a circuit dynamic range.
Consequently, as shown in FIG. 18, amplitude Elpp(s) of an LPP component, which overlaps with a space component of a post-subtraction tracking error signal TE, is also decreased. In this case, the amplitude of the RF residual component Rrp does not change, and therefore the LPP component Elpp(s), which overlaps with the space component, is obscured, making it difficult to be detected. Accordingly, it may fairly be said that in the land pre-pit address demodulating device DMc, the LPP binarized signal Blpp to be outputted from the comparator 5c contains substantially only an LPP binarized signal Blpp(m) corresponding to amark portion, except for few LPP binarized signal Blpp(s) corresponding to a space portion.